1. Field of the Invention
The present invention provides a process for working up the reaction mixture which is obtained in the preparation of polyarylene ether ketones (PAEK) by the so-called nucleophilic route, with isolation of pure PAEK.
2. Description of the Related Art
The preparation of PAEK by nucleophilic polycondensation of bisphenols and organic dihalogen compounds in a suitable solvent using alkali metal carbonates or alkaline earth metal carbonates is a known process which is described in a multitude of patent applications, for example in EP A 0 001 879, EP A 0 182 648 and EP A 0 244 167. When this process is performed, the end of the polycondensation is always followed by a very complicated washing of the reactor effluent. The washing has the aim of removing the reaction solvent, inorganic salts, residual monomers and other compounds from the PAEK. In the course of development studies on PAEK, it has been found that it is advantageous for the process and the product properties when the organic dihalides used are difluoro compounds. However, a disadvantage of the use of difluoro compounds is the formation of sparingly soluble alkali metal or alkaline earth metal fluorides as by-products during the reaction. Moreover, for the solution polycondensation, a suitable solvent has to be present in comparatively large amounts, based on the overall reaction mixture, in order to keep the polymer formed in solution even in the case of relatively high molar masses. For this use, the literature recommends diphenyl sulfone in the vast majority of cases.
By their nature, the abovementioned by-products and the diphenyl sulfone solvent present difficulties in the work-up of the reaction product. All substances other than the polymer have to be removed from the PAEK by complicated washing in various solvents (EP A 0 244 167, EP A 0 182 648, EP A 0 297 363, EP A 0 322 151). These washes correspond additionally to a solid-liquid extraction, since the by-products and the solvent are not only present around but also within the solid polymer; depending on temperature, granule size of the reactor effluent, type of solvent, etc., they require a very large amount of time and large amounts of wash solvent.
In order to separate the PAEK from the by-products and the solvent, two-stage wash processes are typically used. In the first wash step, for example with ethanol, the diphenyl sulfone is removed from the reaction mixture and, in the second wash step with water, the salt-type by-products, generally NaF and KF. According to the prior art, the reactor effluent to be washed is dried in order not to introduce large amounts of water into the wash ethanol. Typically, the individual wash steps proceed at standard pressure and temperatures just below the boiling point of the solvents used. However, a disadvantage is that the residual content of sodium, potassium and diphenyl sulfone can be reduced only insufficiently by the process described, which is disadvantageous for some applications in the electronics industry.
Improved two-stage wash processes are known, with which the residual content of impurities can be reduced even further. Here, the individual wash steps are carried out under elevated pressure, since the solvents are heated above their boiling point. However, these processes are afflicted with the disadvantage that the apparatus configuration is very complicated owing to working at elevated pressure.